The objectives of the study are to assess the contribution of genetics to the variance of behavioral measures, particularly intellectual and motor performance, by correlating scores and measurements of twin, sibling and half-sibling pairs. A report of twin and sibling correlations on the eight-month Bayley Mental Scale has been published. Analyses of four and seven year data are continuing. Bibliographic references: Nichols, P.L. and Anderson, V.E.: Intellectual performance, race and socioeconomic status. Soc. Biol., 20: 367-374, 1973. Nichols, P.L. and Broman, S.H.: Familial resemblance in infant mental development. Dev. Psychol., 10: 442-446, 1974.